Big SWR problem

[Snagger]

Hi again folks.

I have big problems with my SWR readings. On any selected channel, with the meter calibrated to read "set" while transmitting with "FWD" selected, switching to "REF" causes the needle to jump off the right hand side of the scale (I stopped transmitting immediately each time).

I have installed my Midland +98 CB with a roof rack mounted antenna. The rack is a Marslandgalvanised steel expedition type rack, identical to that used on the Camel Trophy Defenders (except for the lack of black paint).

The co-ax used has about 80-90% shielding, and both the core and shileding are copper. The antenna mounting has been attached to the roof rack with a dedicated earth strap, and the rack is very well earthed to the vehicle chassis. The connectors are the screw-in type, but the co-ax core has been soldered in place inside the pins.

I have used my multimeter to try to find a short circuit in the antenna, but it shows no circuit at all between the co-ax core and the shielding or the vehicle earth. I made up a new short co-ax cable to connect the transciever to the SWR meter, just in case the cheap short lead was faulty, but this has not made any difference.

I have been conducting these intitial tests on my driveway, prior to finding somewhere to do a proper tuning run, just to make sure the system works. This puts the antenna about 10' from the house and aluminium garage door. Could that proximity really affect the readings that much, or is there something more fundamental at fault, like a faulty SWR meter?

 

[SilverBullet]

just checking, are there any kinks or coils in your coax?

 

[Tennmogger]

Seeing SWR higher than the meter's "set" reference level certainly says there's a serious problem. Open circuits and shorts can both cause that. A de-tuned antenna, proximity to metal, poor ground at feedpoint can all cause high SWR but usually not as bad as you see. Obviously there cannot be higher reflected signal than there is forward signal but such a simple meter gets confused easy.

In addition to a simple SWR meter, a 50 Ohm dummy load (5 W non-inductive resistor) is a very handy troubleshooting tool (easy to build). A dummy load plugged into the antenna end of your coax would rule out coax cable problems if the SWR was low with the dummy in place.

Sounds like you have verified coax is ok. Trouble spots could be a wallowed out coax female socket in the antenna (you should feel resistance when plugging in the center pin), or it could be a bad antenna.

What kind of an antenna? With a rack mount I'm going to assume it's not a full quarter wave whip (102" thereabouts) so the shortened antenna must have a loading coil either in the middle of the antenna (looks like a big lump, or a tightly wound "distributed" coil around a fiber pole, etc), or at the base. If there's a coil at the base, this might be a shunt fed antenna.

A shunt-fed antenna should show a short to your Ohm meter test, but you said yours showed open. That could be the problem if the antenna is shunt fed.

If not a shunt fed type, the center socket will show open to ground, as you measured, but there should be conductivity from the center socket to the wire/whip on the antenna. If it's an exposed antenna it's easy to measure. If it's totally insulated, try sticking a pin into the wire and measure continuity to that.

Try measuring the SWR at the feedpoint of the antenna, too. That's the proper place to read true antenna response for adjusting the antenna. The measurement of SWR at the feedpoint of the antenna will only be duplicated at the back of the radio if the coax is the right length (half wave at 27 MHz, with velocity factor correction applied). Most of us don't really care if the antenna for a CB is perfect and just tune the entire system for minimum SWR at the radio.

BTW, a coil of coax in the line to the radio really isn't a problem. In fact it can be beneficial because it acts as a choke to RF flowing on the outside of the coax and makes the radio happier.

Bob

PS, I always enjoy seeing references to the Camel Trophy. Lots of newer guys don't know about it. I was a finalist in '82, back in the dark ages.

Hi again folks.

I have big problems with my SWR readings. On any selected channel, with the meter calibrated to read "set" while transmitting with "FWD" selected, switching to "REF" causes the needle to jump off the right hand side of the scale (I stopped transmitting immediately each time).

I have installed my Midland +98 CB with a roof rack mounted antenna. The rack is a Marslandgalvanised steel expedition type rack, identical to that used on the Camel Trophy Defenders (except for the lack of black paint).

The co-ax used has about 80-90% shielding, and both the core and shileding are copper. The antenna mounting has been attached to the roof rack with a dedicated earth strap, and the rack is very well earthed to the vehicle chassis. The connectors are the screw-in type, but the co-ax core has been soldered in place inside the pins.

I have used my multimeter to try to find a short circuit in the antenna, but it shows no circuit at all between the co-ax core and the shielding or the vehicle earth. I made up a new short co-ax cable to connect the transciever to the SWR meter, just in case the cheap short lead was faulty, but this has not made any difference.

I have been conducting these intitial tests on my driveway, prior to finding somewhere to do a proper tuning run, just to make sure the system works. This puts the antenna about 10' from the house and aluminium garage door. Could that proximity really affect the readings that much, or is there something more fundamental at fault, like a faulty SWR meter?

 

[Snagger]

Thanks for the replies.

There are no kinks or coils in the co-ax - I was very careful to avoid them when I ran the cable through the dash and roof lining.

I don't know the name of the type of antenna, but it is a simple thin wire about 30" long (off the top of my head) and has a coil of thicker wire at the bottom. Joining the two is a hexagonal unit with the grub screw to allow length adjustment. That sound like the shunted type, then - the photo below should give you a better idea. It didn't have any kind of connector to fit it to the mount or mate it to the co-ax, and all that Maplins had was an adaptor that fit thorugh the mount with a plastic top disc to isolate the antenne from the mount and a steel washer to make sure the outer casing of the lower portion was earthed. This uses the same co-ax fitting as on the tranciever end. Any advice on that connection would be appreciated, as I bought the kit second hand and it has no instructions.

I get 0 Ohms resistance reading between the shielding of the co-ax at the tranciever end and the vehicle's earth, but as I said, I get a reading of an open circuit even on the multimeter's most sensitive setting for the co-ax core. I haven't checked for a reading between the antenna itself and the tranciever end of the core, but when I made the cable up the resistance showed zero from pin to pin.

I'm beginning to wonder if the SWR meter's switch may have been fitted upside down, so I'm actually reading the reflected signal with FWD selected and trying to callibrate the meter on the reflected signal rather than transmitted. It's the sort of thing that happens to me - I have a knack for suffering rare problems and buying the one faulty part in a batch of thousands.

Most Land Rover enthusiasts in the UK are well aware of the Camel Trophy, and for many of us it's the dream vehicle. I saw a new member on here has a mint condition 300Tdi example, which has to be the best LR ever made.

 

[GlennA]

I would venture to guess that there is a short at the antenna mount. The center conductor is being shorted to ground at the mount itself. Usually there is a nylon/plastic shouldered washer that prevents this from happening.

 

[Snagger]

The plastic washer is fitted. If the antenna was shorting to earth, I would be reading that on the mutlimeter. The problem is not a short.

 

[BigJimCruising]

Just a couple quick thoughts. Don't switch the meter from FWD to REF while transmitting. Unkey then switch then key back up. I'd also be checking the solder connections at both ends of the coax, you might have a cold joint in there. Easy to do, don't ask me how I know that!

 

[tremors834]

I don't know the name of the type of antenna, but it is a simple thin wire about 30" long (off the top of my head) and has a coil of thicker wire at the bottom. Joining the two is a hexagonal unit with the grub screw to allow length adjustment. .

I don't know much about antennas (I just got my Ham license) but are you sure its a CB antenna. Looks like it could pass for a ham antenna with that coil.

 

[Snagger]

I don't know much about antennas (I just got my Ham license) but are you sure its a CB antenna. Looks like it could pass for a ham antenna with that coil.

There are plenty of CB antennas of this design. The thing is, I just measured its length and it's only 760mm long from mount to tip, while the norm for these new seems to be 890mm. Is this likely to be the problem?

Someone suggested to m that my co-ax cable may just happen to be a harmonic length of the wavelength, and that extending it by 18" may make a difference. What do you guys think?

 

[PhulesAU]

unless the coax is the problem(bad) it won'thelp. I'd suggest you use an external meter, NOT the one built into the radio. they are notoriously inaccurate.(cheap) Ifyou look at the firestick website in the tech section theres basically a check list. it might help you run down things to eliminate.

 

[Snagger]

Thanks. I've been looking at the Thunderpole site for help, and like everywhere else it suggests a co-ax short, but I know this is not the cause of my problem. I'm using an external SWR meter (which I have just exchanged for another at the shop just in case the first was faulty).

 

[Snagger]

Well, I just replaced the antenna with a brand new pre-tuned (I know that it still needs fine tuning to the vehicle itself) unit, so the SWR should be within limits, but the fault persists.

So, it's either in the co-ax or the radio itself.

 

[PhulesAU]

What is the saying??? when everything else has been eliminated, what ever is left. No matter how improbable something something. don't be afraid to blame the radio.

 

[tremors834]

Today I ran into a similar problem except it was with a vhf/uhf radio and antenna. I was experimenting with 2 home made dual band vhf/uhf antennas I built. While checking the SWR, the readings were off the chart for both antennas. After investigating (googling) I found out that not all SWR meters are the same. I was using an SWR Meter for CB radio. The meter was only usefull up to 30 Mhz. I need to get an SWR meter that can do VHF(146Mhz) and UHF(440).
Which model meter are you using? Is it possible your SWR meter isn't designed for CB frequencies? Just a thought.

 

[Snagger]

Fantastic feedback (excuse the pun)! It could be something like that.

I made a dummy load from another coax connector and 50 Ohm resistor, and still had problems. So, it's not a co-ax or antenna problem (though the new antenna will still need fine tuning once the main problem has been resolved). Interestingly, a friend from another forum who has a business in South Africa fabricating and instally radio masts and antennas believes it's a problem with the SWR meter too, even though I have already exchanged it for another (identical, possibly from the same batch).

So, at the moment it would seem to be either the SWR meter or the transceiver. I need to find a local club member with a CB and start testing my radio in his vehicle or use his SWR meter.